The goal of this research program is to understand how visual information is encoded by synaptic interactions within neural circuits of the retina. This proposal focuses on the mechanisms and roles for synaptic inhibition mediated by retinal interneurons called amacrine cells. Amacrine cells are the retina's most diverse cell class and the main drivers of functional diversity in retinal circuits: they influence visual processing through their synapses with bipolar cell terminals (excitatory interneurons), ganglion cells (projection neurons), and other amacrine cells. Despite their significance, only a few amacrine cell types have been studied in detail. Over the previous grant period, we discovered and studied novel amacrine cell circuits. We also realized practical and theoretical limitations to using genetic inactivation of individual amacrine cell types to study their roles in retinal function. Building on past experience, here we propose alternative approaches to studying inhibition: conditional deletion of postsynaptic inhibitory receptors at specific points in well-studied retinal circuits, and perturbation of plasmalemmal GABA transporters (GATs). Our preliminary data demonstrate newly-developed mouse genetic tools for conditional knockout (KO) of proteins that are required for functional GABAA or glycine receptors (GABAAR, GlyR). Aim 1 will extend these studies, focusing on gephyrin, which is essential for forming glycinergic synapses, and beta subunits of the GABAAR, which are essential for forming GABAergic synapses. Subsequent experiments will evaluate the structural consequences of protein KO in retinal ganglion cells using super-resolution microscopy (STochastic Optical Reconstruction Microscopy; STORM). We will determine the functional consequences of protein KO on ganglion cell physiology; these experiments will reveal possible mechanisms for compensation following inhibitory receptor deletion versus consequences of inhibitory receptor deletion on receptive fields. Our preliminary data present a completely novel view of GAT-3 function in the mammalian retina. Likely expressed on Mller glial cells, GAT-3 appears to limit presynaptic inhibition of transmission mediated by GABABRs. In Aim 2, we propose optogenetic studies of inhibitory synapses that converge onto ON Alpha or direction-selective ganglion cells. We will test the hypotheses arising from preliminary observations that GAT-3 regulates the strength of synaptic transmission by regulating feedback mechanisms that would otherwise suppress neurotransmitter release. The functional significance of this regulation will be assessed by studies of GAT-3-dependent modulation of receptive field properties in ganglion cell circuits. The proposed studies will advance our understanding of the mechanisms for inhibitory synaptic transmission in the retina and reveal the role of synaptic inhibition in retinal ganglion cell function. The outcome could inform therapies for treating retinal diseases, including retinitis pigmentosa, diabetic retinopathy and glaucoma, all of which are accompanied by changes in inhibitory circuit function.